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Eclipse Day

StarDate:

August 21, 2017

Many skywatchers have been planning for this day for years. The Moon will pass directly between Earth and Sun today, creating a total solar eclipse from Oregon to South Carolina. Day will turn to night, and stars and planets will pop into view. And tens of millions of Americans are expected to see the spectacle in person.

A solar eclipse is the result of a complex ballet involving Earth, the Moon, and the Sun.

The Moon pirouettes against the background of stars and planets every 27 and a half days. But because the Sun is also moving against that background, it takes a couple of days longer for the Moon to catch up to it.

While the Moon passes the Sun roughly once a month, most months it doesn’t cause an eclipse. Instead, it usually glides a little above or below the Sun, because its orbit is tilted relative to the Sun’s path across the sky. A solar eclipse occurs only when the new Moon crosses that path. And even then, most eclipses are only partial: the Moon covers only part of the Sun’s disk, which is what most Americans will see today.

Total eclipses occur an average of about 18 months apart, although the gap can range from a few months to more than two years. The next total eclipse will take place in 2019, visible from parts of South America.

And there’ll be another eclipse over the U.S. on April 8th of 2024, from Texas to Maine. So if you miss out on today’s show, start planning for the next one — another disappearing act for the Sun.

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